[Bug math/19071] New: ldbl-96 lroundl incorrect just below powers of 2

[Bug math/19071] New: ldbl-96 lroundl incorrect just below powers of 2

[jsm28 at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=19071

            Bug ID: 19071
           Summary: ldbl-96 lroundl incorrect just below powers of 2
           Product: glibc
           Version: 2.22
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
          Assignee: unassigned at sourceware dot org
          Reporter: jsm28 at gcc dot gnu.org
  Target Milestone: ---

The ldbl-96 version of lroundl is incorrect for systems with 64-bit long when
the argument's absolute value is just below a power of 2, 2^32 or more, and
rounds up to the next integer; in such cases, it returns 0.  E.g., on x86_64:

Failure: Test: lround (4294967295.5)
Result:
 is:         0
 should be:  4294967296

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[Bug math/19071] ldbl-96 lroundl incorrect just below powers of 2

[cvs-commit at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=19071

--- Comment #1 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
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- Log -----------------------------------------------------------------
https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=cec7d28af8ccb40dbf82d99efa13a040b31b9387

commit cec7d28af8ccb40dbf82d99efa13a040b31b9387
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Mon Oct 5 22:53:50 2015 +0000

    Fix ldbl-96 lroundl just below powers of 2 (bug 19071).

    The ldbl-96 version of lroundl is incorrect for systems with 64-bit
    long when the argument's absolute value is just below a power of 2,
    2^32 or more, and rounds up to the next integer; in such cases, it
    returns 0.  The problem is incrementing the high part of the mantissa
    loses the high bit of the value (which is not an issue for any other
    floating-point format, and is handled specially in lround when the bit
    corresponding to 0.5 was in the high part rather than the low part).

    This patch fixes this in a similar way to that used in llroundl:
    storing the high part in an unsigned long variable before incrementing
    it, so problems cannot occur in the case when this code is reachable.
    I improved test coverage for both lround and llround by making them
    use the same test inputs (appropriately conditioned on the size of
    long in the lround case) - complete with the same comments, to make
    comparison as easy as possible.  (This test coverage improvement was
    how I found the lroundl bug.)

    Tested for x86_64 and x86.

        [BZ #19071]
        * sysdeps/ieee754/ldbl-96/s_lroundl.c (__lroundl): Use unsigned
        long int variable to store possibly incremented high part of
        mantissa.
        * math/libm-test.inc (lround_test_data): Add tests used for
        llround.  Use [LONG_MAX > 0x7fffffff] consistently as condition
        for tests requiring 64-bit long.  Do not condition tests on
        [TEST_FLOAT] unnecessarily.
        (llround_test_data): Add tests used for lround.  Add another
        expectation for the "inexact" exception.  Do not condition tests
        on [TEST_FLOAT] unnecessarily.

-----------------------------------------------------------------------

Summary of changes:
 ChangeLog                           |   12 +++
 NEWS                                |    3 +-
 math/libm-test.inc                  |  170 ++++++++++++++++++++++++++++++++---
 sysdeps/ieee754/ldbl-96/s_lroundl.c |    8 +-
 4 files changed, 178 insertions(+), 15 deletions(-)

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[Bug math/19071] ldbl-96 lroundl incorrect just below powers of 2

[jsm28 at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=19071

Joseph Myers <jsm28 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED

--- Comment #2 from Joseph Myers <jsm28 at gcc dot gnu.org> ---
Fixed for 2.23.

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[Bug math/19071] ldbl-96 lroundl incorrect just below powers of 2

[jsm28 at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=19071

Joseph Myers <jsm28 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |2.23

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